Transferrin and other plasma proteins occur naturally and in abundance within developing CNS and PNS neurons and their processes. Although transferrin is known to be essential for the differentiation and maintenance of sensory neurons in culture, the precise biological role of this plasma protein in neuronal metabolism, development and survival in vivo is unknown. Furthermore, since plasma proteins do not appear to be synthesized by neurons, some type of uptake mechanism must exist whereby these macromolecules can be internalized. For example, studies of non-neural cell types have shown that a variety of important proteins and growth factors including transferrin are internalized by the process of receptor-mediated endocytosis. However, no evidence is currently available on the internalization of transferrin by neurons. Using serum transferrin as a marker protein, we have shown tht transferrin is present in adult and embryonic chicken neurons by immunocytochemical staining with specific antibodies. These studies showed that transferrin is present in greater abundance within peripheral nerve axons and ventral horn neurons of the adult chicken spinal cord as compared to dorsal horn neurons or cerebral cortical neurons. Furthermore, we have purified a protein of MW=56,000 from embryonic chicken neural tissue which represents the neuronal receptor for transferrin. This latter observation strongly suggests that the uptake of transferrin by neurons represents the specific, receptor-mediated process that is observed in other cell types. This research proposal is designed to clarify the role of transferrin in neuronal metabolism, development and survival and to increase our knowledge of the uptake of transferrin by neurons. The specific aims of this proposal are: (A) to study the binding and receptor-mediated uptake of transferrin in cultured spinal cord, cerebral hemisphere and dorsal root ganglion (DRG) neurons in vitro and in the ciliary and superior cervical ganglia in vivo; (B) to characterize the transferrin receptor by physicochemical and immunological means and to determine its neuronal distribution in vivo and in vitro by immunocytochemistry; (C) to determine the role of transferrin in developing neurons in vitro and in developing chickens in ovo. A successful outcome to this proposal will yield basic information regarding the uptake and physiological role of serum proteins in neuronal development. This information may prove vital to understanding such diseases of the motor neuron as amyotrophic lateral sclerosis.